1. Field of the Invention
This invention generally relates to a contour correction apparatus. This invention particularly relates to an apparatus for correcting a video signal to sharpen contours in a picture represented by the video signal. This invention also relates to a method of contour correction.
2. Description of the Related Art
Contour correction apparatuses of various types have been proposed which sharpen contours or edges in pictures represented by video signals. Here, the contours or the edges in the pictures mean the boundaries among different-hue or different-luminance zones in the pictures. A typical contour correction apparatus adds overshoot and preshoot (undershoot) signal components to contour-corresponding or edge-corresponding segments of a video signal.
Japanese published unexamined patent application 5-292522 discloses a color picture quality improving circuit provided with a section for sharpening contours in a picture represented by a video signal.
In the color picture quality improving circuit of Japanese application 5-292522, an input signal "a" including a color-difference signal is deferred by a first delay circuit into a first delayed signal "b". The first delayed signal "b" is deferred by a second delay circuit into a second delayed signal "c". A first maximum level output circuit compares the levels of the signals "a", "b", and "c", and selects one signal from among them which has the maximum level. The maximum-level signal "d" selected by the first maximum level output circuit is fed to a shoot waveform removing circuit. A first minimum level output circuit compares the levels of the signals "a", "b", and "c", and selects one signal from among them which has the minimum level. The minimum-level signal "e" selected by the first minimum level output circuit is fed to the shoot waveform removing circuit.
In the color picture quality improving circuit of Japanese application 5-292522, a first adder combines the input signal "a" and the second delayed signal "c". An attenuator halves the level of the output signal of the first adder, generating the attenuated signal "f". A first inverter acts on the first delayed signal "b". A second adder combines the attenuated signal "f" and the output signal of the first inverter into an addition-resultant signal "g". A second inverter converts the addition-resultant signal "g" into a first contour corrective signal "h". A variable-gain amplifier converts the first contour corrective signal "h" into a second contour corrective signal. A third adder combines the first delayed signal "b" and the second contour corrective signal (the output signal of the variable-gain amplifier) into an addition-resultant signal "i" having suitable preshoot and overshoot components. The third adder outputs the addition-resultant signal "i" to the shoot waveform removing circuit.
In the color picture quality improving circuit of Japanese application 5-292522, the shoot waveform removing circuit includes a second maximum level output circuit and a second minimum level output circuit. The second maximum level output circuit compares the levels of the signals "e" and "i", and selects one signal from among them which has the greater level (the maximum level). The greater-level signal "j" selected by the second maximum level output circuit has a waveform free from lower-side shoot portions (undershoot portions). The greater-level signal "j" is fed to the second minimum level output circuit. The second minimum level output circuit compares the levels of the signals "d" and "j", and selects one signal from among them which has the smaller level (the minimum level). The smaller-level signal "k" selected by the second minimum level output circuit has a waveform free from upper-side shoot portions (overshoot portions). The smaller-level signal "k" is outputted as a correction-resultant signal (a correction-resultant color-difference signal).
The color picture quality improving circuit of Japanese application 5-292522 is unsuited to the processing of a digital video signal.